Currently, a silicon solar cell is widely utilized as one kind of photoelectric conversion device. Photoelectric conversion efficiency of a silicon solar cell has a logical limit of approximately 30%, and this is not such highly efficient energy conversion. This results from the following reasons: (i) near-infrared light whose wavelength is longer than a band edge of silicon (equivalent to wavelength of 1100 nm) does not contribute to photoelectric conversion, (ii) light whose wavelength is shorter than 1100 nm, although absorbed by silicon, is mostly not utilized in photoelectric conversion but converted into heat because of energy relaxation. In order to address the above two problems, it is required to create a photoelectric conversion system that realizes effective utilization of energy in all wavelength regions of the solar spectrum.
So far, for the purpose of effective utilization of solar energy, various developments have been made such as a dye-sensitized solar cell and a tandem solar cell using laminated semiconductors each of which has a different band gap. Such a photoelectric conversion device has been known in which titanium dioxide particles are deposited on a transparent electrode, onto which metal particles are applied and absorbed (see Non Patent Literature 1), and this device realizes photoelectric conversion in the visible light region.
Patent Literature 2 discloses that particles of platinum or palladium are so added to a dye-sensitized solar cell using titanium dioxide as to increase the absorption rate of dye made of ruthenium complex, thereby enhancing photoelectric conversion efficiency.
Meanwhile, various studies have been made on a scheme using titanium dioxide particles with silver or gold particles. Non Patent Literature 1 describes that titanium dioxide particles and gold nanoparticles are used to induce an electron transfer reaction on titanium dioxide particles, so as to realize photoelectric conversion in the visible light region. Non Patent Literature 2 describes that, using titanium dioxide and silver nanoparticles, photocatalysis and photocurrent corresponding to the plasmon band are observed in visible light. In Non Patent Literature 3, a surface photo-voltage (SPV) and a surface photo-current (SPC) in the anatase-type TiO2 particles with which gold is doped are measured so as to analyze charge separation and recombination processes. Patent Literature 1 discloses that a metal surface having a fine projection structure (in a form such as pyramid, stick, cuneiform, dendrite or spherical) is irradiated with light, so that plasmon absorption occurs, in which the irradiation light is absorbed into the metal due to the plasmon resonance, and electrons comes into a higher energy state and are extracted into a semiconductor layer, thereby achieving photoelectric conversion.
Among solar cells utilizing semiconductor substrates made of silicon or the like, it has been studied how to achieve efficient photoelectric conversion at the band edge, using a solar cell basically having PN junctions of semiconductors on which gold nanoparticles or nanostructures are arranged (Non Patent Literature 7, Non Patent Literature 8 and Non Patent Literature 9).